When the LLDPE resins are fabricated into injection-molded products, it is imperative to assure that such products are not susceptible to warping or shrinking. As is known to those skilled in the art, the degree of warping or shrinking can be predicted from the molecular weight distribution of the resins. Resins having a relatively narrow molecular weight distribution produce injection-molded products exhibiting a minimum amount of warping or shrinkage. Conversely, resins having a relatively broader molecular weight distribution produce injection-molded products more likely to undergo warping or shrinkage.
One of the measures of the molecular weight distribution of the resin is melt flow ratio (MFR), which is the ratio of high melt flow index (HLMI or I.sub.21) to melt index (I.sub.2) for a given resin. MFR is defined herein as the ratio of the high load melt index (HLMI or I.sub.21) divided by the melt index (I.sub.2), i.e., ##EQU1## The melt flow ratio is believed to be an indication of the molecular weight distribution of the polymer, the higher the value, the broader the molecular weight distribution. Resins having relatively low MFR values, e.g., of about 20 to about 50, have relatively narrow molecular weight distribution. Additionally, LLDPE resins having such relatively low MFR values produce films of better strength properties than resins with high MFR values.
By comparison, the molecular weight per se of the polymer may be controlled in a known manner, e.g., by using hydrogen. With the catalysts produced according to the present invention, molecular weight may be suitably controlled with hydrogen when the polymerization is carried out at relatively low temperatures, e.g., from about 30.degree. to about 105.degree. C. This control of molecular weight may be evidenced by measurable positive change in melt index (I.sub.2) of the polymer produced.
Another important property of an ethylene and alpha-olefin copolymerization catalyst composition is the ability thereof to effectively copolymerize ethylene with higher alpha-olefins, e.g., C.sub.3 -C.sub.10 alpha-olefins, to produce resins having low densities. Such resins have important advantages, e.g., they are used to produce polyethylene film with excellent physical properties which is, therefore, substantially more resistant to tearing and puncturing than a film made from similar resins of higher densities. This property of the catalyst composition is referred to as "higher alpha-olefin incorporation property" and is usually measured by determining the amount of higher alpha-olefin (e.g., butene, hexene or octene) required in the polymerization process, e.g. fluid-bed reactor process, to produce a copolymer of ethylene and the higher alpha-olefin having a given density. The lesser is the amount of the higher alpha-olefin required to produce a resin of a given density, the higher are the production rates and, therefore, the lower is the cost of producing such a copolymer. Catalysts having good higher .alpha.-olefin incorporation properties are referred to in the art as having a high .alpha.-olefin incorporation factor. High values of the high .alpha.-olefin incorporation factor are especially important in the gas-phase fluid bed process, because relatively high concentrations of higher .alpha.-olefin in the fluid-bed reactor may cause poor fluidization caused, e.g., by resin stickiness. Therefore, production rates must be significantly reduced to avoid such problems. Consequently, catalyst compositions with a relatively high .alpha.-olefin incorporation factor values avoid these problems and are more desirable.
Accordingly, it is important to provide a catalyst composition capable of producing ethylene copolymers having relatively narrow molecular weight distribution (low MFR values) and low densities.
It is therefore a primary object of the present invention to provide a high activity catalyst for the polymerization of alpha-olefins yielding products of a relatively narrow molecular weight distribution.
It is an additional object of the present invention to provide a catalytic process for polymerizing alpha-olefins which yields linear low density polyethylene of a relatively narrow molecular weight distribution at high productivity.